1. Field of the Invention
The present invention relates to a holographic memory medium and a recording apparatus using this medium.
2. Description of the Related Art
A volume holographic memory system is one of digital information recording systems which use the principle of holography. This volume holographic memory system converts digital data or an information signal to a two-dimensional dot beam pattern with a contrast, adds a reference beam to this pattern, yielding an optical interference pattern, and records this optical interference pattern as two-dimensional data on a recording medium. For the recording medium is used a photorefractive crystal such as LN which has a cylindrical shape, a rectangular parallelepiped or the like.
As reproducing an information signal, the reference beam alone is irradiated on the recording medium to thereby reproduce an optical interference pattern and performs inverse transform on the optical interference pattern, yielding a dot beam pattern. The original data can be acquired by reconverting this signal to digital data by using an electronic circuit.
Because the same reference beam as used in the recording operation is irradiated on a recording medium in a reproducing operation, recorded information may be erased at the same time as information is reproduced. As one solution to such a problem, the beam power of the reference beam to be used in a reproducing operation is set lower than that needed in a recording operation. However, repeating the reproducing operation erases recorded information slightly though it occurs. In this respect, there has been a demand for a system which has less reproduction deterioration.
A two-color holographic memory system performs recording and reproducing operations using a combination of lights of different wavelengths by utilizing the intermediate level of optical excitation in a recording medium which is comprised of a photorefractive crystal. Specifically, a gating beam whose wavelength differs from that of the reference beam is used in such a way that information is recordable only when this gating beam and the reference beam are both irradiated on the recording medium. A reproducing operation is carried out by irradiating the reference beam alone on the recording medium while cutting off the gating beam. This prevents reproduction deterioration.
In the above two-color holographic memory system, if the gating beam is irradiated on the recording medium at a position different from a predetermined target position where information is to be recorded, the information signal recorded at the irradiated portion may be erased.
For instance, as the gating beam irradiated on the recording medium passes through the recording medium and emerges from the opposite side, part of the gating beam may be reflected at the emerging surface of the recording medium and may return inside the recording medium. At the surface of the recording medium where the gating beam has been irradiated, part of the gating beam may be reflected there and further reflected by the components of the recording apparatus to reach the recording medium again. Those stray lights may undesirably erase non-target recorded signals.
Accordingly, it is an object of the present invention to provide a holographic memory medium which can prevent generation of stray light of a gating beam at the time of recording information, and a recording apparatus which uses this memory medium.
To achieve the above object, according to one aspect of this invention, there is provided a holographic memory medium for recording an information signal carried by a signal beam as a coherent reference beam and the signal beam enter the memory medium with a gating beam being present, which memory medium has at least a first surface and a second surface parallel to each other and comprises an antireflection coat, formed on the first surface, for preventing reflection of the gating beam.
According to another aspect of this invention, there is provided a recording apparatus for recording an information signal using a holographic memory medium, which comprises light absorbing member of absorbing the gating beam coming through the memory medium.